Perfusion refers to the flow of blood into and out of the tissue capillary bed. Quantification of tissue perfusion is of interest to clinicians across many surgical and non-surgical specialties. Although simple binary assessment (flow versus no-flow) may be adequate for some clinical applications, quantification of perfusion in standard measures is desirable in many other clinical applications. To date, quantitative assessment of tissue perfusion has remained elusive.
Photoplethysmography (PPG) is an optical technique that can be used to estimate changes in microvascular blood volume, and PPG-based technology has been deployed in commercially available medical devices for assessing pulse rate, oxygen saturation, blood pressure, and cardiac output. A typical output of such devices is the PPG waveform that corresponds to the heartbeat of the subject. Despite the relatively wide application of PPG technology to such medical devices, PPG has not been utilized to provide measurements in standardized units when assessing blood flow. A PPG technology with such capabilities would enable routine measurements of blood flow in tissue, including perfusion measurements, to be made in standardized units of volume/unit time/tissue area. This would be of significant value to clinicians, as such measurements would allow direct inter-site and inter-subject comparisons.